/*
 * Copyright (c) Kumo Inc. and affiliates.
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace melon {
    namespace detail {
        template<typename T>
        template<typename Tag, typename VaultTag>
        struct SingletonHolder<T>::Impl : SingletonHolder<T> {
            Impl()
                : SingletonHolder<T>(
                    {typeid(T), typeid(Tag)}, *SingletonVault::singleton<VaultTag>()) {
            }
        };

        template<typename T>
        template<typename Tag, typename VaultTag>
        inline SingletonHolder<T> &SingletonHolder<T>::singleton() {
            return detail::createGlobal<Impl<Tag, VaultTag>, void>();
        }

        [[noreturn]] void singletonWarnDoubleRegistrationAndAbort(
            const TypeDescriptor &type);

        template<typename T>
        void SingletonHolder<T>::registerSingleton(CreateFunc c, TeardownFunc t) {
            std::lock_guard<std::mutex> entry_lock(mutex_);

            if (state_ != SingletonHolderState::NotRegistered) {
                /* Possible causes:
                 *
                 * You have two instances of the same
                 * melon::Singleton<Class>. Probably because you define the
                 * singleton in a header included in multiple places? In general,
                 * melon::Singleton shouldn't be in the header, only off in some
                 * anonymous namespace in a cpp file. Code needing the singleton
                 * will find it when that code references melon::Singleton<Class>.
                 *
                 * Alternatively, you could have 2 singletons with the same type
                 * defined with a different name in a .cpp (source) file. For
                 * example:
                 *
                 * Singleton<int> a([] { return new int(3); });
                 * Singleton<int> b([] { return new int(4); });
                 *
                 * Adding tags should fix this (see documentation in the header).
                 *
                 */
                singletonWarnDoubleRegistrationAndAbort(type());
            }

            create_ = std::move(c);
            teardown_ = std::move(t);

            state_ = SingletonHolderState::Dead;
        }

        template<typename T>
        void SingletonHolder<T>::registerSingletonMock(CreateFunc c, TeardownFunc t) {
            if (state_ == SingletonHolderState::NotRegistered) {
                detail::singletonWarnRegisterMockEarlyAndAbort(type());
            }
            if (state_ == SingletonHolderState::Living ||
                state_ == SingletonHolderState::LivingInChildAfterFork) {
                destroyInstance();
            } {
                auto creationOrder = vault_.creationOrder_.wlock();

                auto it = std::find(creationOrder->begin(), creationOrder->end(), type());
                if (it != creationOrder->end()) {
                    creationOrder->erase(it);
                }
            }

            std::lock_guard<std::mutex> entry_lock(mutex_);

            create_ = std::move(c);
            teardown_ = std::move(t);
        }

        template<typename T>
        T *SingletonHolder<T>::get() {
            if (MELON_LIKELY(
                state_.load(std::memory_order_acquire) ==
                SingletonHolderState::Living)) {
                return instance_ptr_;
            }
            createInstance();

            if (instance_weak_.expired()) {
                detail::singletonThrowGetInvokedAfterDestruction(type());
            }

            return instance_ptr_;
        }

        template<typename T>
        std::weak_ptr<T> SingletonHolder<T>::get_weak() {
            if (MELON_UNLIKELY(
                state_.load(std::memory_order_acquire) !=
                SingletonHolderState::Living)) {
                createInstance();
            }

            return instance_weak_core_cached_.get();
        }

        template<typename T>
        std::shared_ptr<T> SingletonHolder<T>::try_get() {
            if (MELON_UNLIKELY(
                state_.load(std::memory_order_acquire) !=
                SingletonHolderState::Living)) {
                createInstance();
            }

            return instance_weak_core_cached_.lock();
        }

        template<typename T>
        melon::ReadMostlySharedPtr<T> SingletonHolder<T>::try_get_fast() {
            if (MELON_UNLIKELY(
                state_.load(std::memory_order_acquire) !=
                SingletonHolderState::Living)) {
                createInstance();
            }

            return instance_weak_fast_.lock();
        }

        template<typename T>
        template<typename Func>
        invoke_result_t<Func, T *> detail::SingletonHolder<T>::apply(Func f) {
            return f(try_get().get());
        }

        template<typename T>
        void SingletonHolder<T>::vivify() {
            if (MELON_UNLIKELY(
                state_.load(std::memory_order_relaxed) !=
                SingletonHolderState::Living)) {
                createInstance();
            }
        }

        template<typename T>
        bool SingletonHolder<T>::hasLiveInstance() {
            return !instance_weak_.expired();
        }

        template<typename T>
        void SingletonHolder<T>::preDestroyInstance(
            ReadMostlyMainPtrDeleter<> &deleter) {
            instance_copy_ = instance_;
            deleter.add(std::move(instance_));
        }

        template<typename T>
        void SingletonHolder<T>::destroyInstance() {
            if (state_.load(std::memory_order_relaxed) ==
                SingletonHolderState::LivingInChildAfterFork) {
                if (vault_.failOnUseAfterFork_) {
                    KLOG(DFATAL) << "Attempting to destroy singleton " << type().name()
                            << " in child process after fork";
                } else {
                    KLOG(ERROR) << "Attempting to destroy singleton " << type().name()
                            << " in child process after fork";
                }
            }
            state_ = SingletonHolderState::Dead;
            instance_core_cached_.reset();
            instance_.reset();
            instance_copy_.reset();
            if (destroy_baton_) {
                constexpr std::chrono::seconds kDestroyWaitTime{5};
                auto const wait_options =
                        destroy_baton_->wait_options().logging_enabled(false);
                auto last_reference_released =
                        destroy_baton_->try_wait_for(kDestroyWaitTime, wait_options);
                if (last_reference_released) {
                    vault_.addToShutdownLog("Destroying " + type().name());
                    teardown_(instance_ptr_);
                    vault_.addToShutdownLog(type().name() + " destroyed.");
                } else {
                    print_destructor_stack_trace_->store(true);
                    detail::singletonWarnDestroyInstanceLeak(type(), instance_ptr_);
                }
            }
        }

        template<typename T>
        void SingletonHolder<T>::inChildAfterFork() {
            auto expected = SingletonHolderState::Living;
            state_.compare_exchange_strong(
                expected,
                SingletonHolderState::LivingInChildAfterFork,
                std::memory_order_relaxed,
                std::memory_order_relaxed);
        }

        template<typename T>
        SingletonHolder<T>::SingletonHolder(
            TypeDescriptor typeDesc, SingletonVault &vault) noexcept
            : SingletonHolderBase(typeDesc), vault_(vault) {
        }

        template<typename T>
        bool SingletonHolder<T>::creationStarted() {
            // If alive, then creation was of course started.
            // This is flipped after creating_thread_ was set, and before it was reset.
            if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
                return true;
            }

            // Not yet built.  Is it currently in progress?
            if (creating_thread_.load(std::memory_order_acquire) != std::thread::id()) {
                return true;
            }

            return false;
        }

        template<typename T>
        void SingletonHolder<T>::createInstance() {
            if (creating_thread_.load(std::memory_order_acquire) ==
                std::this_thread::get_id()) {
                detail::singletonWarnCreateCircularDependencyAndAbort(type());
            }

            std::lock_guard<std::mutex> entry_lock(mutex_);
            if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
                return;
            }
            if (state_.load(std::memory_order_relaxed) ==
                SingletonHolderState::LivingInChildAfterFork) {
                if (vault_.failOnUseAfterFork_) {
                    KLOG(DFATAL) << "Attempting to use singleton " << type().name()
                            << " in child process after fork";
                } else {
                    KLOG(ERROR) << "Attempting to use singleton " << type().name()
                            << " in child process after fork";
                }
                auto expected = SingletonHolderState::LivingInChildAfterFork;
                state_.compare_exchange_strong(
                    expected,
                    SingletonHolderState::Living,
                    std::memory_order_relaxed,
                    std::memory_order_relaxed);
                return;
            }
            if (state_.load(std::memory_order_acquire) ==
                SingletonHolderState::NotRegistered) {
                detail::singletonWarnCreateUnregisteredAndAbort(type());
            }

            if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
                return;
            }

            SCOPE_EXIT {
                // Clean up creator thread when complete, and also, in case of errors here,
                // so that subsequent attempts don't think this is still in the process of
                // being built.
                creating_thread_.store(std::thread::id(), std::memory_order_release);
            };

            creating_thread_.store(std::this_thread::get_id(), std::memory_order_release);

            auto state = vault_.state_.rlock();
            if (vault_.type_.load(std::memory_order_relaxed) !=
                SingletonVault::Type::Relaxed &&
                !state->registrationComplete) {
                detail::singletonWarnCreateBeforeRegistrationCompleteAndAbort(type());
            }
            if (state->state == detail::SingletonVaultState::Type::Quiescing) {
                return;
            }

            auto destroy_baton = std::make_shared<melon::Baton<> >();
            auto print_destructor_stack_trace =
                    std::make_shared<std::atomic<bool> >(false);

            // Can't use make_shared -- no support for a custom deleter, sadly.
            std::shared_ptr<T> instance(
                create_(),
                [destroy_baton, print_destructor_stack_trace, type = type()](T *) mutable {
                    destroy_baton->post();
                    if (print_destructor_stack_trace->load()) {
                        detail::singletonPrintDestructionStackTrace(type);
                    }
                });

            // We should schedule destroyInstances() only after the singleton was
            // created. This will ensure it will be destroyed before singletons,
            // not managed by melon::Singleton, which were initialized in its
            // constructor
            SingletonVault::scheduleDestroyInstances();

            instance_weak_ = instance;
            instance_ptr_ = instance.get();
            instance_core_cached_.reset(instance);
            instance_.reset(std::move(instance));
            instance_weak_fast_ = instance_;
            instance_weak_core_cached_.reset(instance_core_cached_);

            destroy_baton_ = std::move(destroy_baton);
            print_destructor_stack_trace_ = std::move(print_destructor_stack_trace);

            // This has to be the last step, because once state is Living other threads
            // may access instance and instance_weak w/o synchronization.
            state_.store(SingletonHolderState::Living, std::memory_order_release);

            vault_.creationOrder_.wlock()->push_back(type());
            vault_.instantiatedAtLeastOnce_.wlock()->insert(type());
        }
    } // namespace detail
} // namespace melon
